1. Field Of The Invention
This invention relates to the field of seating systems and more particularly to the field of seating systems for wheelchairs and other applications including where it is desirable to distribute the supporting pressure substantially uniformly throughout the entire engagement area between the user's body and the underlying support structure of the seating system.
2. Discussion Of The Background
It is known and well documented that persons confined to wheelchairs for extended periods of time are subject to and at risk of developing decubitus ulcers or pressure sores. These pressure sores normally occur in the tissue surrounding or adjacent to the bony prominences of the skeletal structure and are due to the pressure applied to them by the weight of the seated person. That is, the tissue between the seated person's bones and the seating surface is normally squeezed under the user's weight. As a result, the normal flow of blood through the capillary vessels to the squeezed tissue areas can become occluded or blocked. Continued pressure and the resulting lack of blood flow to the tissue will cause necrosis (dead tissue) to form and in the most severe cases may cause an open wound from the epidermis (skin) all the way down through the fatty and muscular tissue to an exposed bone. Unattended, the decubitus ulcers (pressure sores, wounds) may become infected and eventually may even become fatal to the seated person. The most common areas of pressure sore development are the ischial tuberosities and the coccyx (tailbone).
There have been numerous prior art approaches developed to specifically deal with the associated problems of confined wheelchairs users. A cushion by Roho, Incorporated which consists of an array of upstanding inflatable bulbs has been partially successful in minimizing the effects of pressure on the seated wheelchair user. The Roho cushion basically attempts to equalize the interface pressure across the entire surface of the user in contact with the supporting cushion. The cushion is inflated just to the point where the user is suspended off the bottom of the structure so that the user will not bottom out. The desired result is that the peak pressures which normally occur at the supported skeletal structure are reduced to an average pressure defined by the supported weight of the user divided by the net surface area in contact with the cushion.
While this type of invention may appear theoretically optimal to alleviating the effects of interface pressure on the seated user, the Roho cushion, as originally designed, does not address the need for proper posture and positioning. These are equally critical to the continued health of the wheelchair bound user as is the avoidance of pressure sores. The design of the Roho cushion inherently introduces an amount of undesirable instability. This instability is caused by the communication of air throughout the entire array of inflated bulbs. Thus, the Roho cushion responds to asymmetrical loading or dynamic loads due to mobility requirements of the user in an equal and opposite direction as would be expected. This type of response will aggravate existing asymmetrical postures and in some cases create undesirable seated postures. Prolonged abnormal posture will eventually cause permanent, fixed deformities in the spine.
The original cushion by Jay Medical, Ltd. (U.S. Pat. No. 4,588,229) was developed to address both the problems of pressure and positioning. The original Jay cushion consists of a relatively rigid tray in which a seating well is formed in the rearward portion of the cushion to accept a flexible, sealed bladder or pad containing a viscous fluid. The fluid bladder or pad has a seal along the longitudinal centerline of the cushion which creates two equal side segments to the pad. There are additional seals within the perimeters of these two segments to limit the maximum overall thickness that the pad may achieve in these local areas when the fluid contents are pressurized by the user's seated weight. The resulting assembly is a seating system that not only reduces the interface pressures at the sensitive bony prominences but also provides a stable support surface from which proper postural position can be achieved. This is true regardless of any existing postural asymmetries or dynamic loading caused by mobilization of the wheelchair.
The seam along the longitudinal centerline of the Jay pad specifically prevents an asymmetrical condition from getting worse (i.e., prevents a user's existing pelvic obliquity from bottoming out on the relatively rigid tray). The viscous fluid generates the required amount of pressure equalization under and around the primary bony prominences at risk while seated in the wheelchair. At the same time, it dampens the dynamic response due to mobility of the user. The theoretical deficiency with the Jay design is that because of the longitudinal segmentation of the pad into distinct sides, it may be possible to generate higher interface pressures on one ischial tuberosity than on the other. This is true because the fluid is not able to communicate from one side to the other and thus cannot average the pressures across the entire surface as does the prior art Roho cushion. The design of the original Jay cushion does have the ability to bias or create higher than average pressures at anatomical regions known to be able to sustain increased pressure at reduced risk of tissue breakdown. In this regard, the elevated front portion of the Jay cushion serves such a purpose. That is, the user' s thighs (or femurs) in the Jay cushion are supported by a lesser amount of fluid than are the user's buttocks where support is primarily the viscous fluid bladder or pad and a lesser amount of rigid tray material. The end result is that a greater than normal proportion of the user's weight is bearing on the thighs. This reduces the remaining weight portions which must be supported by the user's buttocks which contain the high risk breakdown tissue areas (ischial tuberosities and coccyx).
Later embodiments of the basic Roho cushion design addressed the aforementioned problem by segmenting the array of inflatable bulbs in a manner that allowed the user to over inflate the thigh areas and under inflate the sensitive areas under the buttocks (see U.S. Pat. No. 4,698,864).
A problem was identified with the Jay cushion when used in certain circumstances. If the seated user did not maintain a correct seated posture due to fatigue, poor back position, neurological impairments (e.g., the user slouches in the seat) or the user was placed in a semi-reclined position, the bulk of the fluid in the rear of the cushion would migrate forwardly or rearwardly into any unrestrained areas of the fluid pad. The net effect was that the user's coccyx or tailbone would fail to be supported by any fluid and would come to bear on the underlying relatively rigid tray. This was an undesirable condition for these type of wheelchair users. A design modification was implemented to reduce the risk of bottoming out the cushion when subjected to the circumstances defined above. A second seal was made laterally across the rear half of the fluid pad. This lateral seal essentially trapped a predefined amount of fluid in the rear area of the pad thus preventing the average user from bottoming out his coccyx when reclined or when slouched in the wheelchair. The additional segmenting did, however, further limit the communication of fluid around the buttocks area. Interface pressure measurements compared between the original Jay cushion and the cushion with the lateral seam revealed increased average pressure on the ischial tuberosities due to the added seam. Thus, the overall pressure reducing capabilities of the cushion were sacrificed to prevent bottoming out onto the rigid tray.
Further use and examination of the two Jay cushion configurations identified an additional performance deficiency. On certain users who were particularly bony due to, for example, muscle atrophy or weight loss, the users would continue to bottom out their coccyx on the rigid portion of the cushion. It was determined that the longitudinal seam down the centerline of the Jay pad was contributing to the user's ability to work their coccyx down between the fluid segments on either side of centerline. It was discovered that these problems could be resolved either by overfilling the rear quadrants of the lateral seam pad to prevent bottoming out or by incorporating a dedicated segment or fluid pouch for the coccyx into the pad design. The latter was chosen for the present invention in lieu of overfilling the rear quadrants of the lateral seam pad which could result in higher average cushion interface pressures due to lack of immersion of the user.